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The invention relates to the modulation of light beams and in particular to modulating light using light valves based on deformable mirrors.
Light valves, also referred to as spatial light modulators, have found use in many different fields. One particular industrial field in which these devices have made great impact is the printing industry, where they are extensively used with lasers to create lithographic printing plates. Often the lasers are of near-infrared wavelengths.
Light valves, or linear and two dimensional arrays of light valves, are typically employed when a large number of light spots have to be individually modulated. The imagewise exposure of a printing plate is a good example of such an application wherein pixel size and dot resolution are critical parameters. The computer-to-plate technology, in particular, has made great demands upon the performance of these devices in that the limits of optical power handling, switching speed and resolution are all under pressure due to the operational demands of this industry.
Another application field that stands to benefit from this technology is that of optical communications. In this field there is an ever-increasing need for devices that may be used to switch, modulate or process light signals.
One particular subset of light valves is based on the reflection of the incident light from micro-miniature deformable mirrors. Prior art deformable mirror light valves can be generally divided into three types:
a. Cantilever or hinged mirror type light valves which deflect the light when bending or tilting. A well-known example in this category is the DMD technology developed by Texas Instruments.
b. Membrane light valves where a flat membrane is deformed into a concave or spherical mirror, focusing the light.
c. Grating light valves which diffract the light by forming a periodic physical pattern. A well-known example in this category is the Grating Light Valve developed by Silicon Light Machines of Sunnyvale, Calif. This technology is described by Bloom in his paper in Proc. SPIE-Int. Soc. Opt. Eng. (USA) vol.3013 p.165-p.171. This type of technology is also referred to as xe2x80x9ccyclic recording systemsxe2x80x9d in earlier literature.
Considerable effort has been invested internationally in the in the field of silicon microelectromechanical systems (MEMS) and in the improvement of the fabrication and operation of devices of these three types in particular. Great technical achievements have been obtained in this regard and many process steps have been improved to obtain remarkable device yields, but a number of central limitations remain in respect of these devices.
The major disadvantage of the hinged or cantilevered mirror type devices is comparatively slow response time. Typically, response times in these devices are of the order of 10 microseconds. This is due to the low natural frequency of a cantilever mirror and the large deflection required. Typical cantilevers are 10 microns to 5 microns long. This requires the tip to deflect between 1 micron and 5 microns in order to deflect the light about 10 degrees. Such large deflections reduces the response time of the device.
Membrane light valves have the advantage of relatively higher speeds. However, their major disadvantage is difficulty in fabrication, as the membrane is supported around its complete periphery making it difficult to fabricate the cavity under it by micromachining. Micromachining is the most desirable fabrication method for deformable mirror light valves as it uses standard processes that have been developed for the manufacturing of integrated circuits. The difficulty in fabricating a monolithic membrane device of this type hinges on the fact that it requires a multi-step process where micromachining and bonding of a membrane is required. An example of such a membrane device is shown by Hornbeck in U.S. Pat. No. 4,441,791. The device cannot be fabricated out of a single piece of silicon because of the membrane.
Grating light valves are very fast and response times below 100 nanoseconds are attainable. The major disadvantage of the grating light valves is the low optical efficiency inherent to optical diffraction gratings. Grating light valves may be applied using the zero order diffracted beam or the first order diffracted beam. For zero order use, the contrast ratio is poor. For first order use, the optical efficiency is low, as each first order beam contains less than 50% of the energy. This can be improved by a method known as xe2x80x9cblazingxe2x80x9d. In this method the ribbons in a grating light valve are tilted, or the multiple ribbons forming one spot are progressively deflected to an increasing extent. While this can overcome some of the light loss, it still requires multiple ribbons for each light spot. The devices of this nature are based upon the concept embodied by Bloom in U.S. Pat. No. 5,311,360.
In accordance with the present invention, an array of individually addressable micromachined light reflecting ribbons provide a high speed light valve action by employing a method in accordance with which the ribbons flex about their longer axis under the action of an applied electrostatic field. The ribbons are mounted on pedestals such that the resulting micromechanical structures are substantially symmetrical with respect to a plane along their long axis through the centers of the pedestals and normal to the substrate. This micromechanical arrangement ensures a structure with a high natural frequency, thereby allowing high light valve switching speeds. The particular mode of deformation also ensures a robust structure. The method may be applied in either xe2x80x9cbright fieldxe2x80x9d or xe2x80x9cdark fieldxe2x80x9d (Schlieren) mode. The particular combination of materials employed allows the method to be used in higher incident power applications such as those encountered in the printing industry. It also finds application in optical communications. The invention uses the fabrication methods developed for grating light valves to build a deformable mirror light valve combining the fabrication and speed advantages of the former with the simplicity of the latter. Since the ribbons flex about their long axes, the amplitude of tip deflection required to produce a given angular light deflection is much smaller than that required by prior art cantilever devices, which flex about their short axes.